Immunological determination of the order of folding of portions of the molecule during air oxidation of reduced ribonuclease

An immunological method is used to follow the folding of different portions of the reduced bovine pancreatic ribonuclease molecule during air oxidation. Antibodies that react specifically with segments 1-13, 31-79, and 80-124 of native ribonuclease, as they are folded, were purified by affinity chromatography, using antiserum to native ribonuclease and columns to which the ribonuclease fragments were attached. The kinetics of reaction between these prufied antibodies and refolded portions that are produced when reduced rebonuclease is oxidized by air demonstrate the presence of intermediate states of folding, and are consistent with folding of the anti-genic determinants in the order 80-124, 1-13, and 31-79. The relative stabilities of each of these segments to thermal denaturation in the native protein provide additional evidence that the native conformation of region 80-124 is a very stable one in the intact molecule. On the basis of these two types of evidence, it appears that segment 80-124 contains a nucleation site for the folding of the protein molecule.